fix: some open AF_UNIX sockets in forked child processes

[boquan-fang]

Resolved issues:

Partially solve #4005

Description of changes:

• Use s2n_io_pair_close and s2n_io_pair_close_one_end to close AF_UNIX sockets in tests that have fork().
  • Child processes are often forgot to be cleaned up.

Call-outs:

• See PR#4833 and PR#4835 for fixing other open fds.
• I didn't add a test in this PR. The test to detect all opened fds will be in a separate PR.

Testing:

• Test locally.
  • Add --track-fds=yes to CTest memcheck and direct all Valgrind output to MemoryTester.log files.
  • Search in VS Code for Open File Descriptors of AF_UNIX.
    • Previously opened AF_UNIX sockets in tests that are fixed are gone.
  • This is similar to the check in ./codebuild/bin/test_exec_leak.sh.
    

    s2n-tls/codebuild/bin/test_exec_leak.sh

    Lines 82 to 97 in 6bb195c

    	# run valgrind with track-fds enabled
    	valgrind_log_dir=valgrind_log_dir
    	for test_file in detect_exec_leak detect_exec_leak_finish; do
    	LD_LIBRARY_PATH="build/lib:$TARGET_LIBCRYPTO_PATH/lib:$LD_LIBRARY_PATH" S2N_VALGRIND=1 \
    	valgrind --leak-check=full --show-leak-kinds=all --errors-for-leak-kinds=all \
    	--run-libc-freeres=yes -q --gen-suppressions=all --track-fds=yes \
    	--leak-resolution=high --undef-value-errors=no --trace-children=yes \
    	--suppressions=tests/unit/valgrind.suppressions --log-file="build/$valgrind_log_dir/$test_file" \
    	build/bin/$test_file
    	# search for all leaked file descriptors, excluding the valgrind_log_dir file
    	cat build/$valgrind_log_dir/$test_file | \
    	grep "Open file descriptor" | \
    	grep --invert-match $valgrind_log_dir \
    	&& fail "file leak detected while running $test_file"
    	done

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.